My question is how do we know whether to say Ittaqullah and not Attaqullah or Uttaqullah.

Question
Bismillah
Assalamu Alaikum wa Rahmatullahi wa Barakatuh
Kindly explain the rules of how to read a word when one needs to seperate it from the word before it for example I know that when we seperate Ya Ayyuhallazina Amanuttaqullaha and want to say 'fear Allah' on its own, we say Ittaqullaha. My question is how do we know whether to say Ittaqullah and not Attaqullah or Uttaqullah. I remember learning the rule long ago, something to do with the sign (fathah, dhamma, kasra) on the next letter, but have since forgotten.
Please explain with examples from the Qur'an.
Jazak Allahi Kkhair

Answer
Wa alaikum assalaam wa rahmatullahi wa barakatuh,
The rules involved in the question are the hamzah al-wasl rules. We have to look at the word beginning with the hamzah al-wasl and determine whether it is a verb, a noun, or a participle. In this case, the word is a command form of a verb. In a verb starting with hamzah al-wasl, we look at the vowel on the third letter, if it is a fathah or a kasrah, then we start the hamzah al-wasl with a kasrah.
To see the detailed lessons on hamzah al-wasl, please see the following link. You will find many examples in the eight different lessons insha' Allah:http://www.abouttajweed.com/hamzah-al-wasl/index.php